With up-to-date source:

#   compile  z/infback.o
/usr/BUILD/tools/bin/i386--netbsdelf-gcc -O2 -Wall -Wmissing-prototypes -Wstrict-prototypes -Os -ffreestanding   -Werror -mcpu=i386  -I/usr/src/sys/arch/i386/stand/boot/biosboot/../../../../..//lib/libz/../../../common/dist/zlib -I /usr/src/sys/arch/i386/stand/boot/biosboot/.. -I /usr/src/sys/arch/i386/stand/boot/biosboot/../../lib -I /usr/src/sys/arch/i386/stand/boot/biosboot/../../../../..//lib/libsa -I /usr/BUILD/obj/sys/arch/i386/stand/boot/biosboot -nostdinc -D_STANDALONE -I/usr/src/sys/arch/i386/stand/boot/biosboot/../../../../../ -DSUPPORT_PS2 -DDIRECT_SERIAL -DSUPPORT_SERIAL=boot_params.bp_consdev -DCONSPEED=boot_params.bp_conspeed -DCONSADDR=boot_params.bp_consaddr -DCONSOLE_KEYMAP=boot_params.bp_keymap -DSUPPORT_CD9660 -DSUPPORT_USTARFS -DSUPPORT_DOSFS -DPASS_BIOSGEOM -DPASS_MEMMAP -DEPIA_HACK -I/usr/src/sys/arch/i386/stand/boot/biosboot/../../../../..//lib/libkern/../../../common/lib/libc/quad -I/usr/src/sys/arch/i386/stand/boot/biosboot/../../../../..//lib/libkern
 /../../../common/lib/libc/string -I/usr/src/sys/arch/i386/stand/boot/biosboot/../../../../..//lib/libkern/../../../common/lib/libc/arch/i386/string -nostdinc -isystem /usr/BUILD/netbsd/usr/include  -D_ZLIB_PRIVATE   -nostdinc -isystem /usr/BUILD/netbsd/usr/include -c    /usr/src/sys/arch/i386/stand/boot/biosboot/../../../../..//lib/libz/../../../common/dist/zlib/infback.c -o infback.o
/usr/src/common/dist/zlib/infback.c:183:1: "BITS" redefined
In file included from /usr/src/common/dist/zlib/zutil.h:24,
                 from /usr/src/common/dist/zlib/infback.c:15:
		 /usr/src/sys/lib/libkern/libkern.h:178:1: this is the location of the previous definition